How to universally skip database touches when precompiling assets on Heroku

add this to config/application.rb

config.assets.initialize_on_precompile=false                                                  

took me a while to hunt this down... adding it to config/environments/*.rb did NOT work

UPDATE: It doesn't work with rails 4

Heroku now makes a labs flag available that'll make the runtime environment available during compilation time, which means your app will be able to successfully connect to your DATABASE_URL database.

First you need to install the labs plugin:

$ heroku plugins:install http://github.com/heroku/heroku-labs.git

then enable the user-env-compile labs feature:

$ heroku labs:enable user-env-compile --app your-app-name

For me the problem is activerecord calling instantiate_observer in lib/active_record/railtie.rb:92. This will load the observers and the respective models. has_and_belongs_to_many then connects to the db.

I think I'll override this method when ENV["RAILS_ASSETS_PRECOMPILE"] is present, which is used by devise in the fix Bradley linked to.

EDIT: So this snippet fixed it for me:

namespace :assets do  # Prepend the assets:precompile_prepare task to assets:precompile.  task :precompile => :precompile_prepare  # This task will be called before assets:precompile to optimize the  # compilation, i.e. to prevent any DB calls.  task 'precompile_prepare' do    # Without this assets:precompile will call itself again with this var set.    # This basically speeds things up.    ENV['RAILS_GROUPS'] = 'assets'    # Devise uses this flag to prevent connecting to the db.    ENV['RAILS_ASSETS_PRECOMPILE'] = 'true'    # Prevent loading observers which will load the models which in turn may hit    # the DB.    module ActiveModel::Observing::ClassMethods      def instantiate_observers; end    end    # Prevent route drawing because certain gems might get called which will hit    # the DB.    class ActionDispatch::Routing::RouteSet      def draw; end    end  endend